<?xml version="1.0"?> <xsl:stylesheet version="1.0"                 xmlns:xsl="http://www.w3.org/1999/XSL/Transform"                xmlns:pre="http://www.example.com">  <xsl:template match="/">    <!-- unavailable extension function is a must signal error;         processors cannot recover -->    <root><xsl:value-of select="pre:fn()"/></root>  </xsl:template></xsl:stylesheet>